Sp2300

The optical imaging of a single AuNP on an MoS₂ monolayer-covered gold film was measured by an inverted dark-field microscope (Eclipse Ti-U, Nikon), which was equipped with an oil-immersed dark-field condenser (NA = 1.20 to 1.43), a water-immersed objective lens (NA = 0.6, 40×, part number MRH08430), and a quartz tungsten halogen lamp (LV-LH50PC, Nikon) as the light source. The dark-field scattering spectrums of a single AuNP on MoS₂ monolayer-covered gold film were captured by a grating spectrometer (Acton Spectra Pro SP-2300, Princeton Instruments) equipped with a liquid-nitrogen-cooled charge-coupled device (CCD). The extinction spectra of individual AuNPs on MoS₂ monolayer surfaces were measured by a spectrometer (SP-2-300, Princeton Instruments) and an electron-multiplying charge-coupled device (EMCCD) camera (ProEM+, Princeton Instruments). The electrochemical measurements were carried out with three-electrode system by an Autolab (Autolab PGSTAT302N, Metrohm AG). The square step potential can be applied on the sample and the corresponding current of the entire electrode is recorded by the Autolab. The Ag/AgCl wire and Pt coil were used as the reference electrode and the counterelectrode, respectively. The MoS₂ monolayer-covered gold film was the working electrode. The electrolyte is 0.1 M NaF aqueous solution.

The optical measurements were performed on a home-built confocal microphotoluminescence system (fig. S17). A single wire was locally excited with a 351-nm laser beam (BeamLok 2065, Spectra-Physics) focused by an objective lens (50×; numerical aperture, 0.8; Nikon CFLU Plan). The emission of the wire was selectively collected from the tip using a confocal microscopy setup with a 1-μm pinhole. The light was subsequently coupled to a grating spectrometer (Acton SP2300) with a matched thermal electrically cooled charge-coupled device (ProEm: 1600 × 200B, Princeton Instruments).
For the measurements of electric field–induced asymmetric light propagation, an external electric field was applied with a commercial dc voltage supplier. The pulsed electric field was applied using a nanosecond signal generator, which supports tunable amplitude, frequency, and rise time (fig. S18). The optical signals were detected using a silicon photodetector and monitored using an oscilloscope to obtain the response time and switching frequency of the switch.

For time-resolved femtosecond TA measurements, the fundamental beam produced by the Yb:KGW laser (Pharos, 1030 nm, 20 W, 100 kHz; Light Conversion Ltd.) was separated into several light beams and sent to a homebuilt microscopic ultrafast pump-probe spectrometer collinearly and then lastly combined and focused by a microscope to a spot size of 1 μm. For the pump beam, one of the seed beams was introduced into a commercial noncollinear optical parametric amplifier (2H-NOPA, Light Conversion Ltd.) and a second harmonic generation crystal beta-barium borate (BaB2O4) to generate the pump light with a certain wavelength centered at 470 nm. For the probe beam, another seed beam was focused onto an yttrium-aluminum-garnet crystal (8 mm thickness) to generate continuum white light as the probe light. The temporal delay time between the pump beam and the probe beam was controlled via a high-resolution motorized delay stage (M-ILS250HA, Newport). The pump and probe pulses overlapped spatially at the sample, and the transmitted probe light was collected by a spectrograph (SP2300, Princeton Instruments) coupled to a nitrogen-cooled detector (PyLon100B, Princeton Instruments). The transient transmission signal was calculated by contrasting and normalizing the transmitted probe spectra with and without a pump beam: ΔT/T = (T_pump − T_unpump)/T_unpump.